Add HAVE_*_H checks around inclusions of netinet/in.h and sys/socket.h

Only include <netinet/in.h> and <sys/socket.h> if the system supplies them.
This is necessary to try and get the mingw32 build working again.

No need to include <sys/types.h> and <netinet/in.h> from cdns.c - these are
already included by <irc.h>.

include/irc_std.h

@@ -37,7 +37,9 @@
 /*
  * Everybody needs these INET headers...
  */
+#ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
+#endif
 #ifdef HAVE_ARPA_INET_H
 #include <arpa/inet.h>
 #endif